kinetic and thermodynamic processes on weak acid … · 2017. 7. 18. · cr3+ cation predominates...
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Annals of the Academy of Romanian Scientists
Online Edition Series on Chemistry Sciences
ISSN 2393 – 2902 Volume 2, Number 1/2015 34
KINETIC AND THERMODYNAMIC PROCESSES
ON WEAK ACID RESINS IN METALS REMOVAL
FROM WASTEWATER
Cristina COSTACHE1, Cristina MODROGAN
1, Giani APOSTOL
1
Abstract. The chemical and metallurgical industries are the main producers of metal-bearing liquid
effluents that due to the toxic character of many of the metals contained in them need to
be treated before the final discharge. The removal of Cr6+
from aqueous solution by an
ion-exchange resin is described. Ion-exchange resins Purolite A 500 shows a remarkable
increase in sorption capacity for Cr6+
, compared to other resins. The sorption process,
which is pH dependent shows the maximum removal of Cr6+
in the pH range 1–5 for an
initial Cr6+
concentration of 1 g/L. The sorption of Cr6+
on these cation exchange resins
follows first-order reversible kinetics and pseudo-first-order kinetics.
Key words: ion-exchange; heavy metals; liquid effluents; environment, Cr6+
Introduction
Chromium, occurring as Cr3+
or Cr6+
in the environment, is an important
material resource, an essential micronutrient or toxic contaminant too. Cr3+
is
necessary for the normal development of human and animal organisms but Cr6+
activates teratogenic processes, disturbs DNA synthesis and can give rise to
mutagenous changes leading to malignant tumours (Cavaco et al., 2009). Natural
sources of chromium include weathered rocks, volcanic exhalations and
biogeochemical processes and, in the man-polluted environment, mainly waste
after processing and utilization of chromium compounds. Chromium is an
important and widely used element in industry. The hexavalent and trivalent
chromium are often present in electroplating wastewater. Other sources of
chromium pollution are leather tanning, textile, metal processing, paint and
pigments, dyeing and steel fabrication. To remove toxic chromium compounds
from sewages several methods are used such as: precipitation, coagulation,
solvent extraction and various kinds of membrane processes, ion flotation,
adsorption and ion-exchange (Dąbrowski et al., 2004). The maximum limit in
drinking water is of 0.05 mg/L. The drinking groundwater chromium content
ranges on the average from 0.07 to 2 mg/L.
1 University Politehnica of Bucharest, Department of Analytical Chemistry and Environmental
Protection, Corresponding author: [email protected]
Kinetic and Termodinamic Processes on Weak Acid Resins
in Metals Removal from Wastewater 35
0.02 mg/L is accepted as the permissible content of chromium in groundwater.
The daily dose taken by the adult can be of 50-200 mg/day (or 60-290 mg/day).
Cr3+
cation predominates in most tissues except the liver. Chromium is associated
with nucleic acids and it may be concentrated in liver cells. It plays an important
role in the metabolism of glucose, certain proteins and fats, is part of enzymes and
stimulates the activity of others. All chromium compounds without chromate are
rapidly cleared from the blood. Chromium also accumulates in the liver and
kidneys. High concentrations of chromium, observed in the lungs of people
exposed to this metal, indicate that at least part of chromium is stored in this organ
in the form of insoluble compounds.
The binding of chromium with the elements of blood and transport of
chromium by blood depends mainly on its valence. Hexavalent chromium readily
crosses the membranes of red blood cells and after reduction to trivalent
chromium is bound to hemoglobin. The reduction of hexavalent to trivalent
chromium, occurring within cells, considered as the activation of the carcinogenic
chromium, increases because of the probability of interaction of trivalent
chromium with the DNA.
Among various techniques, the adsorption is the most promising technique to
remove and eliminate Cr6+
from drinking water. Some of the conventional
techniques for metals removal from industrial wastewater include chemical
precipitation, adsorption, solvent extraction, membrane separation, ion-exchange,
electrolytic techniques, coagulation/flotation, sedimentation, filtration, membrane
process, biological process and chemical reaction (Asgari et al., 2008; Gode and
Pehlivan, 2003).
Sorption operations, including adsorption and ion-exchange are a potential
alternative water and wastewater treatment. In an adsorption process, atoms or
ions (adsorbate) in a fluid phase diffuse to the surface of a solid (adsorbent),
where they are bound with the solid surface or are held there by weak
intermolecular forces (Kocaoba and Akcin, 2002). A number of investigators have
studied the removal of metal ions namely cadmium, cobalt, zinc, silver, copper,
mercury, chromium and lead from aqueous solution using different adsorbents
(Anderson, 1997; Boddu et al., 2003; Lee et al., 2003; Park et al., 2008; Verma et
al., 2006). However, the improved sorption capacity of ion exchange resins may
have advantages over such non-specific adsorbents and so the ion-exchange resins
present great potential for the heavy metals removal from water and industrial
wastewater. Rengaraj and co-workers reported on the removal of heavy metals
from water and nuclear power plant coolant water using ion exchange resins
(Alonso, 1999; Boddu et al., 2003; Dupont and Guillon, 2003; Gandhi et al.,
2010; Mahvi et al., 2010; Rivero et al., 2004; Sengupta, 1995; Verma et al., 2006).
36 Cristina COSTACHE, Cristina MODROGAN, Giani APOSTOL
In the present study, the anion exchange resin A 500 was used for the removal
of Cr6+
from an aqueous solution. The main objective of this study was to
investigate the kinetic parameters of this ion-exchange resin. In addition, the
parameters that influence the process, such as initial Cr6+
concentration, contact
time, pH, and kinetic studies were also investigated.
The application of ion-exchange is an attractive method to remove heavy
metal contaminants. Nowadays, chelating resins are increasingly used in the
removal of metal ions due to their high adsorption capacities, selectivity and
durability (Alonso, 1999; Ahmaruzzaman and Sharma, 2005; Yang and Al-Duri,
2005; Ortiz et al., 2003). It obtained very good results in the sorption and
separation of traces and milligram amounts of Cr6+
complexes using chelating ion-
exchangers. These resins are polymers with the functional groups able to form
complexes with selected ions. In contrast to the conventional ion-exchange resins,
chelating resins combine with ion-exchange and complex formation and hence
can exhibit high selectivity for some ions or groups of ions, providing a wide
range of practical use and industrial applications both in hydrometallurgy and
analytical chemistry. The properties of chelating ion-exchangers depend on the
type of functional groups, though to a lesser extent on grain size and physical
properties. Rivero et al. (2004) studied the Cr6+
removal from wastewater using
Lewatit FO36, an anion –exchanger. The ion-exchange resins are used in their
entire range to improve water quality, depending on the types and concentrations
of the impurities. The selective resins were deemed major options for wastewater
treatment. Traces of environmentally harmful substances accumulate on the
Lewatit FO36 ion-exchange resins and are thereby removed from the wastewater.
They can be used for the elimination or separation of heavy metals from aqueous
solutions, accumulation and recovery of heavy metals, removal of heavy metals
from water and process water. Lewatit FO36 has high exchange capacity, very
good chemical, mechanical and thermal stability, good ion-exchange kinetics
make it suitable for the treatment of electroplating rinse waters. Gel resins usually
have higher efficiencies and cost less (Rivero et al., 2004).
Experimental
Cr6+
can exist in several forms such as Cr2O7 2-
, HCrO4 - , HCrO7
- and CrO4
2-
and the relative abundance of particular complex depends on the concentration of
the chromium ion and the pH of the solution. In the concentration range and pH of
the work the main species of Cr6+
is HCrO4 −, as shown in Fig 1.
Kinetic and Termodinamic Processes on Weak Acid Resins
in Metals Removal from Wastewater 37
Fig.1. Predominance diagram showing the relative distribution of different Cr
6+
species in water as a function of pH and total Cr6+
concentration (Rivero et al.,
2004)
The following anion-exchange resin A 500 Purolite was used in this study for
the removal of Cr6+
from wastewater. Purolite A500 is a macroporous type I
strong basic anion-exchanger having quaternary ammonium groups. The
characteristic properties of this ion-exchanger used are presented in Table 1.
Table 1. The characteristic properties of ion-exchanger (Technical Data Sheet Purolite A500)
Property Ion-exchange resin type Purolite A500
Polymer structure macroporous polystyrene crosslinked with
divinylbenzene
Physical form and appearance Faint light yellow spherical beads
Functional groups Type I Quaternary ammonium R-N(CH3)3OH
Ionic form as shipped Cl-
Particle size range 0.3 – 1.2 mm
Moisture retention 53 – 58%
Uniformity coefficient 1.7
Total exchange capacity 1.15 eq/L
Specific gravity 1.08
Operating temperature 100ºC
38 Cristina COSTACHE, Cristina MODROGAN, Giani APOSTOL
Fig.2. FTIR spectra of (a) A 500 resin and (b) chromium-sorbed A 500 resin
(Rivero et al., 2004)
Figure 2 (a and b) depicts FTIR spectra of fresh A500 resin and chromium
treated A500 resin respectively. The broad band at 3440 cm-1
indicates the
presence of –OH and –NH stretching vibrations. The band at 2924 cm-1
indicates
the –CH stretching vibration in –CH and –CH2. A band at 1635 cm-1
corresponds
to – NH bending vibration in –NH2 and 1065 cm-1
indicates the presence of –CO
stretching vibration in –COH. The band at 1079 cm- 1
, 1052, 862 cm-1
suggesting
that N-methyl glucamine present in the A500 resin and in chromium sorbed A500
resin these bands become broad and shifted to lower frequencies which confirms
that chromium sorption has occurred on the A500 resin. The new band at 906 cm-1
indicates the Cr =O bonds of chromate anion confirming the sorption of Cr6+
onto
the A500 resin. The new band at 540 cm-1
indicates the presence of Cr(OH)3 in
the chromium treated A500 resin (Mahvi et al., 2010).
All the chemicals used were of analytical grade and were produced by Merk,
Germany. A stock solution of 1 g/L of Cr6+
was prepared by dissolving 14.13 g of
K2Cr2O7 in 1000 mL distilled water. This solution was diluted as required to
obtain standard solution containing 0.1 –1.0 g/L of Cr6+
.
Kinetic studies were carried out with different initial concentrations of Cr6+
while maintaining the resin dosage at a constant level. In batch experiments, 100
mL of sample solution and ion-exchanger (1.0 g) were put into a conical flask and
shaken at different time intervals using a laboratory shaker. These experiments
indicated that no adsorption by the container walls was detectable. In the kinetic
study, aqueous solutions containing 0.1, 0.4, 0.6, 0.8 and 1.0 g/L were used. 100
Kinetic and Termodinamic Processes on Weak Acid Resins
in Metals Removal from Wastewater 39
mL of solution were contacted with 1 ± 2·10-4
g of resin, in batch system, at
contact times of 2, 4, 6, 8, 10, 15, 20, and 30 minutes. The shaking speed was of
200 rpm to maintain resin particles in suspension. The influence of the pH was
studied on a resin saturated with Cl- as exchange ion, at pH values of 1.0 ± 0.05,
2.0 ± 0.05 3.0± 0.05 and 5 ± 0.2, at temperature of 20 ± 1°C. Temperature control
was done by using an incubator FOC 225E –Velp Scientifica. Cr6+
concentration
in aqueous phase was determined by the colorimetric method based on diphenyl-
carbazyde as colour reagent, using a spectrophotometer model CINTRA 5. The
experiments were conducted in three parallel series.
To describe the ion exchange equilibrium, it was presumed that it can be
mathematically described as adsorption equilibrium. Adsorption isotherms were
drawn up with different initial concentrations varying from 0.1g/L to 1.0 g/L of
metal ions while keeping the constant amount of resins at room temperature 20oC.
The equilibrium between the solid and liquid phases was modelled by the
Langmuir and Freundlich equations as it follows.
Results and discussion
In this paper, the ion-exchange kinetics was studied by recording variation in
time of Cr6+
concentration in aqueous phase. In this way, integral kinetic curves
concentration vs. time C(t) were drawn. To determine variation in time of
chromium concentration in resin, a mass balance equation was used
(Ahmaruzzaman and Sharma, 2005). (Eq. 1), where: V is volume of aqueous
phase (mL); Ci is Cr6+
concentration in aqueous phase before phase contact
(mg/L); Ce is Cr6+
concentration in aqueous phase at equilibrium (mg/L); q is Cr6+
concentration in resin at equilibrium (mg/kg); m is resin mass (g).
m
CCVq ei )( (1)
The study of ion-exchange in order to characterise the process from the kinetic
point of view comprises the curves obtained using the experimental apparatus
described above. The process efficiency is controlled by the kinetics of adsorption
and hence the several kinetic models are available to predict the mechanism
involved in the sorption process. Among these models, there are pseudo-first-order,
pseudo-second-order and interparticle diffusion, rate equation (Boddu et al., 2003).
The process efficiency is controlled by the kinetics of adsorption and hence
several kinetic models are available in order to predict the mechanism involved in
the sorption process. Among these models, Lagergren’s rate equation appears to
be one solute from a liquid solution (Yang and Al-Duri, 2005).
Pseudo-first-order (Eq. 2):
40 Cristina COSTACHE, Cristina MODROGAN, Giani APOSTOL
tk
qqq ete303,2
loglog 1 (2)
Pseudo-second-order (Eq. 3):
22
1
eqke
q
t
tq
t (3)
The interparticle diffusion model has been applied to various adsorption systems
and was found to account for adsorption systems (Eq. 4) (Yang and Al-Duri,
2005).
5,0tkq pt (4)
Samples were taken after 2.5, 5, 10, 20 and 30 minutes, the results of the
experiments are presented in Figs. 2-5. The curves reveal that the ion-exchange
rate is higher using Purolite A500, with a higher exchange capacity.
Fig. 2. Rate of sorption in resin vs.
retention time at pH =1
Fig.3. Rate of sorption in resin vs.
retention time at pH =2
Kinetic and Termodinamic Processes on Weak Acid Resins
in Metals Removal from Wastewater 41
Fig.4. Rate of sorption in resin vs.
retention time at pH =3
Fig.5. Rate of sorption in resin vs.
retention time at pH = 5
Figures 2-5 show the different variations of Cr6+
concentration in time. The
results showed that Cr6+
removal was more than 95.6% at a contact time of 30
min. These diagrams depict the effect of pH on chromium removal. The
maximum efficiency of chromium removal is at pH=1. The result shows that the
increasing the value of pH in wastewater sample, the extent of removal increases.
The pH of the solution plays a very important role in the chromium sorption.
The amount of retained chromium is the highest at pH 1.0, for a solution
containing 1 g/L of Cr6+
. The amount of the retained chromium decreases rapidly
when the pH is increased above 5 due to the formation of a chromium precipitate
at higher pH values. For the ion-exchange resins, the sorption at pH above 5
shows a decreasing trend because of the formation of hydroxyl complexes of
chromium, Cr(OH)3 (Mahvi et al., 2010; Ortiz et al., 2003).
The similar behaviour between the two samples in terms of kinetics in the first
5 minutes of the ion-exchange process is probably due to the fact that the ion-
exchange process is controlled by internal diffusion. Several kinetic models
including pseudo first order, pseudo second order and pore diffusion were used
for the simulation of the experimental data. The kinetic parameters calculated
from equations 2-5 for the adsorption of Cr6+
are given in Table 2. The pseudo
second order and pore diffusion models successfully fit the adsorption kinetic.
42 Cristina COSTACHE, Cristina MODROGAN, Giani APOSTOL
Table 2. Kinetic model parameters for the adsorption of Cr6+
on A500
Kinetic model parameters Temperature,
20oC
A 500
Pseudo first order k1 0.033
r2 0.999
Pseudo second order k2 0.235
r2 0.986
Pore diffusion kp 5.949
r2
0.991
Pseudo first order adsorption kinetic model experiments were also directed in
an attempt to understand the kinetics of Cr6+
removal by the A500 Purolite resins.
It is a well-established fact that the sorption of ions in an aqueous system follows
reversible pseudo first order kinetics, when a single species is considered on a
homogeneous surface. The kinetics of sorption describing the solute uptake rate
which, in turn, governs the residence time of the sorption reaction is one of the
important characteristics that define the sorption efficiency.
Sorption involves the transfer and resulting equilibrium distribution of one or
more solutes between a fluid phase and solid particles. The design of this kind of
system requires: first, knowledge of the sorption equilibrium that can be obtained
from experimental data. It is also important to determine the working capacity of
an ion-exchanger. Secondly, as these operations usually take place in fixed beds,
some knowledge of how transitions travel through a bed is required. This
introduces both time and space into the analysis. Methods for analyzing fixed bed
transitions go from the local equilibrium theory, based only on stoichiometric
factors, to full rate modelling where mass transfer resistances are incorporated.
The equilibrium between Cr6+
in the solution and in the anion exchanger
A500, in the Cl− form, is described by Eq. 5. The equilibrium normally cannot be
described by a simple binary approach but anion exchanging resins in acidic pH
are very highly selective for chromate.
The Langmuir equation was applicable to the homogeneous adsorption system,
while the Freundlich equation was the non-empirical one employed to describe
the heterogeneous systems and was not restricted to the formation of the
monolayer. The well-known Langmuir equation was represented as given by Eq.
(5), where: qe is the equilibrium Cr6+
ions concentration on the ion-exchanger,
(mg/g); Ce is the equilibrium Cr6+
ions concentration in solution (mg/L); qm is the
monolayer capacity of ion-exchanger (mg/g); b is the Langmuir adsorption
constant (L/g) related to the free energy of adsorption.
Kinetic and Termodinamic Processes on Weak Acid Resins
in Metals Removal from Wastewater 43
e
eme
Cb
Cbqq
1 (5)
The values of qm and b were calculated from the slope and the intercept of the
linear plots Ce/qe vs. Ce. On the other hand, the Freundlich equation was
represented by Eq. (6), where: Kf and 1/n are the Freundlich constants
corresponding to the adsorption capacity and the adsorption intensity.
n
efe CKq /1 (6)
The plot of ln qe vs. ln Ce was employed to generate the intercept Kf and the
slope 1/n. The equilibrium isotherm, the relation between the amount exchange
(qe) and the remaining concentration in the aqueous phase (Ce), is important to
describe how solutes interact with the resins and thus is critical in optimising the
use of the resins. Cr6+
sorption isotherms of A500 resin is presented in Fig. 6 as a
function of the equilibrium concentration of metal ion in the aqueous medium at
room temperature (20°C) for 24 hours of contact time. The amount of Cr6+
ions
adsorbed per unit mass of the resin increased with the initial metal concentration
as expected. To obtain maximum sorption capacities or reach the plateau values
that represent the saturation of the active groups, which are available for
interaction with Cr6+
ions on the resin, the initial concentration was increased from
0.1 to 1.0 g/L of Cr6+
. The resin was saturated at relatively low concentrations
indicating strong binding for Cr6+
.
Fig.6. The isotherm Langmuir at
different pH
Fig.7. The isotherm Freundlich at
different pH
Figures 6 and 7 show the comparison between experimental and simulated
data using the Langmuir and Freundlich isotherms with the parameters given in
44 Cristina COSTACHE, Cristina MODROGAN, Giani APOSTOL
Table 3. Both isotherms can be classified as favourable (concave downward)
which means that a considerable amount of solute can be achieved in the solid
although there is a low concentration in the fluid phase. Besides, the value of n of
the Freundlich isotherm lies between 1 and 10, which also indicates favourable
adsorption. The values of the parameters reported in Table 3 show that the best
fitting of the experimental data was obtained with the Freundlich equation with
fitting parameters that fall in the range reported in the literature. However, there is
insufficient improvement to discard the simplicity of the chemical equilibrium
constant approach.
Table 3. Parameters of Langmuir and Freundlich equations and chemical equilibrium
pH Langmuir Freundlich
a, mmole/g b,
L/mmole
R2
K, mmole/g n R2
1 2.398 4.799 0.880 1.803 5.155 0.996
3 2.134 4.644 0.866 1.800 5.078 0.990
5 2.188 4.664 0.802 1.788 5.021 0.960
Conclusions
In the present study, the removal of Cr6+
using A500 Purolite was found to be
effective. The experimental results indicated that A500 can be successfully used
for the adsorption of Cr6+
from aqueous solutions using a batch system. The
results herein indicate that ion-exchange resins can efficiently remove traces of
Cr6+
present in aqueous solutions.
The adsorption of Cr6+
on ion exchange resins followed intraparticle diffusion
process. We conclude that ion-exchange resins could be exploited for applications
in the tertiary level treatment of drinking water as well as industrial effluents. The
kinetic data would be useful for the design and built wastewater treatment plants.
The use of the resin A500 led to viable results for the treatment of
groundwater polluted/contaminated with Cr6+
. The equilibrium between the resin
and the liquid phases was also properly described by the empirical Freundlich
isotherm.
Ion-exchange of the Cr6+
is dependent on the initial concentrations of the
adsorbate, contact time and pH level. Experimental parameters must be optimally
selected to obtain the highest possible removal of Cr6+
from aqueous solutions
using A500. The amount of Cr6+
ions adsorbed per unit mass of A500 increased
with the initial chromium concentration as expected. To reach the plateau values
that represent the saturation of the active groups which are available for
interaction with Cr6+
ions on the resin.
Kinetic and Termodinamic Processes on Weak Acid Resins
in Metals Removal from Wastewater 45
The equilibrium data well followed the linear Langmuir and Freundlich
models. The adsorption of the Cr6+
ion is pH dependent. The results obtained from
the effect of pH on the adsorption capacity of the A500 indicated that the
maximum ion-exchange was obtained at pH 1 for resin. The total ion-exchange
capacities for Cr6+
ions are in the range of 0.12–0.55 mmole/g for A 500.
Acknowledgements
The present work has been funded by the Sectorial Operational Programme Human
Resources Development 2007-2013 of the Ministry of European Funds through the
Financial Agreement POSDRU/159/1.5/S/134398.
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